1. Field of the Invention
The present invention relates to a device for sensing a velocity of a moving object. The device may be provided in an optical disk device, for example, to sense a velocity of an optical beam spot relative to an optical disk.
2. Description of the Related Art
In an optical disk device, a seek operation in which an optical head is moved from one track to another track on an optical disk must be carried out at high speed and with great accuracy. Namely, an optical beam spot radiated onto an optical disk should be moved at high speed and be positioned exactly at a predetermined track of the disk. The reliability of the seek operation largely depends upon the quality of a signal indicating the relative velocity between the optical beam spot and each of tracks on the disk.
Conventionally, the relative velocity has been sensed as follows.
A beam reflected from the disk is detected by two photo detectors disposed at positions adjacent to each other. Then, a difference between two signals outputted from the two photo detectors, i.e., a difference signal, is obtained. In the seek operation, since the optical beam spot is moved in a radial direction of the disk and alternately traverses lands (i.e., tracks) and grooves formed on the disk, the beam reflected from the disk changes periodically, and thus, the difference signal also changes periodically.
Namely, the waveform of the difference signal is determined in accordance with the velocity of the optical beam spot relative to the disk, and the inclination of the waveform is in proportion to the velocity of the optical beam spot. In other words, although the inclination does not show the velocity directly, the inclination has a value which is equal to a differential value obtained by differentiating the waveform with respect to time. Accordingly, the velocity is obtained by multiplying the differential value by a proportional coefficient which changes in accordance with the positional relationship between the optical beam spot and the track.
Thus, the proportional coefficient changes at every position of the disk, and the waveform of the difference signal does not show the moving velocity of the optical beam spot directly. Therefore, in a conventional device, it is difficult to obtain the velocity of the optical beam spot at high speed and with great accuracy.